Actuators of this kind are known, for example, from U.S. Pat. No. 6,476,702. The actuators contain a mass-spring system which can oscillate and which is made to oscillate when an alternating current is driven through the electrically conductive coil. The actuators are used for an extremely wide variety of purposes, for example as linear electromagnetic actuators (linear motors) in pumps, as oscillation generators or as oscillation absorbers. In the last-mentioned case, an actuator of the kind described in the introductory part is mechanically operatively connected to an oscillating component and oscillations are generated in the actuator, these oscillations being superimposed on the oscillations of the component. Given suitable selection of amplitude, frequency and phase of the oscillations which are generated in the actuator, the oscillations of the component are reduced or absorbed.
In the simplest case, the abovementioned linear electromagnetic actuator includes only two parts, specifically a coil which is wound around a core and a magnet which is embedded in a ferromagnetic casing, wherein one of the two constituent parts of the actuator is mounted in a sprung manner in relation to the other stationary constituent part. Despite this simple construction, high excitation forces can be generated within the actuator since, in addition to the electrodynamic forces which are created when an alternating current is driven through the electrically conductive coil, rectified magnetic reluctance forces also have an effect in the actuator. To this end, collar-like projections are provided on the core of the coil and on the casing of the magnet in such a way that the magnetic flux across these projections is shut off via the air gap which is located between the projections, and the reluctance forces act at these locations.
In linear electromagnetic actuators of this kind, the transmission behavior of the system, as a matter of principle owing to the design of the actuator as an oscillating mass-spring system, inter alia has a resonant peak at the first natural frequency fe (resonant frequency) of the system. This is dependent on the stiffness of the mass suspension means and the size of the moving mass. The oscillating mass “builds up” in an undesired manner when “passing through” this resonant frequency fe or in the region of the resonant frequency fe. This may result in very poor control properties and also, under certain circumstances, in the oscillating mass loudly striking the actuator housing. This currently leads to a restriction in the frequency operating range of the actuator system because the operating range of the actuator is shifted toward relatively high frequencies in order to not excite the resonant frequency fe during operation. The (expedient) minimum of the first natural frequency fe of a linear electromagnetic actuator for damping oscillations in a motor vehicle is typically between approximately 40 and 60 Hz, depending on the design and configuration.
However, for a typical application, in a vehicle, of an actuator of this kind, for example as an active oscillation absorber, effectiveness at a low frequency, for example in the idling range of the vehicle, is desirable and, sometimes, also required. Consequently, there is a need to set the first natural frequency fe of the actuator below the idling frequency of the vehicle motor.
The resonant frequency fe of a simple mass-spring system can be reduced by means of reducing the spring stiffness and/or by means of increasing the oscillating mass. Since the spring system used in the linear electromagnetic actuator also has to compensate for the magnetic transverse stiffness between the coil and the magnet, a further reduction in the spring stiffness is not possible or only possible to a very slight extent. Therefore, it is necessary to make the adjustment by means of increasing the size of the oscillating masses. However, increasing the oscillating mass in the linear electromagnetic actuator also entails an increase in the total weight and a considerable increase in the size of the installation space.
Reduction of the lower natural frequency fe below, for example, the idling range of the vehicle motor also results in the problem of increased excitation of this low-frequency resonance fe due to, for example, carriageway unevennesses.
Irrespective of this, it is desirable in mass-spring systems of this kind to damp the resonant deflections. In this case, the actuator can be operated in a relatively large frequency range and also in the resonant range when the resonant peak in the transmission function is damped to a sufficient degree.